Hi Don !
One way of perserving the pattern, whilst simplifying the definition would be to extend the range in a symmetrical way (see below).
Define an SDC quite simply as a DL-ALS in which one ALS is a bivalue.
Looking for the pattern would then involve taking a bivalue and seeing whether in its box, row or column it double-links to another ALS.
In this way, the historic name remains, the scope increased, symmetry emphasised and the definition simplified.

At present SDCs :
- begin in a box (bivalue double-linking in box to second ALS)
- end in a row or column (remaining cells of second ALS).
Just complete the symmetry and include those which
- begin in a row or column
- end in a box (note if beginning in a row or column, then by virtue of the double-links, the second ALS is necessarily in a box, so no other configuration is possible)

That would, as you say, require definition of DL-ALS, but then unlike any other advanced concept, DL-ALS could be explained to a novice in 10 minutes including a coffee break !

A good point and interesting concept, but it's too late for me since I find the SDCs so easily now. Off the top of my head I can see the DL-ALS approach practical with patterns comparable to basic SDCs, but wonder if that would be true for patterns comparable to so-called extended SDCs. But that's only speculation.

At present we have a complicated definition for a specific example (SDC) of a simple concept (DL-ALS), itself a specific example of a vast concept (Rank 0 logic).
As discussed recently HSR is another specific example of that vast concept which unlike DL-ALS cannot be simply described.

If SDC were simple to explain, this conversation would be almost pointless, but the whole point is that if the greater concept is easier to explain than the specific instance, why on earth not stay simple all the way : simple concept, simple specific example.
Indeed starting from the simple concept, the extensions to the specific example, earlier mentioned, would, I think, appear natural.